The movement of bacteria toward or away from chemicals (chemotaxis) is important for cell growth and survival and may also play a major role in the progression of many infectious diseases. Escherichia coli is the pre-eminent model system for understanding the mechanisms underlying transmembrane signaling, motility and cellular behavior. E. coli chemotaxis/motility pathway is one of the best-studied sensory signal transduction systems in biology, yet it remains to be fully understood at the molecular level. Our long-term goal is to elucidate the structural basis of chemotactic signaling and flagellar switchin in living cells. The overall objective of this proposal is to correlate structural changes in the chemoreceptor complex and the flagellar motor of cells in different signaling states. By imaging flagellated minicells (~ 0.2 um diameter) generated from a skinny mreB mutant, we will be able to use cryo-electron tomography to determine high-resolution structures of the receptor array and flagellar motor trapped in different signaling states within the same individual minicells. Innovative microfluidic techniques will enable us to record the behavior of these minicells. Structure and function can be observed under identical conditions in an unprecedented manner. Thus, we will be able to test hypotheses about how changes in the structure of receptor arrays and motor complexes are manifested as changes in behavior.